Glass can be generally
divided into two groups: oxide glass and non-oxide glass. The ingredients of
oxide glasses include oxides (chemical compounds that include oxygen).
Non-oxide glasses are made from compounds that contain no oxides, and which
often instead contain sulfides or metals. Oxide glasses are much more widely
used commercially. The common types of glass discussed below are all oxide
glasses.

Soda-lime glass
is the kind of glass used for flat glass, most containers and electric light
bulbs, and many other industrial and art objects. More than 90 percent of all
glass is soda-lime glass. It has been made of almost the same materials for
hundreds of years. The composition is about 72 percent silica (from sand), about
13 percent sodium oxide (from soda ash), about 11 percent calcium oxide (from
limestone), and about 4 percent minor ingredients. Soda-lime glass is
inexpensive, easy to melt and shape, and reasonably strong.

All glass container
manufacturers use the same basic soda-lime composition, making the containers
easy to recycle. Manufacturers sort the glass by color and then later reuse it
in the production of new containers.

Soda-lead glass,
commonly called crystal or lead glass, is made by substituting
lead oxide for calcium oxide and often for part of the silica used in soda-lime
glass. Soda-lead glass is easy to melt. It is much more expensive than soda-lime
glass. Soda-lead glass has such beautiful optical properties that it is widely
used for the finest tableware and art objects. In addition, lead oxide improves
the electrical properties of glass.

Borosilicate glass
is heat-shock resistant and better known by such trade names as Pyrex and Kimax.
It contains about 80 percent silica, 4 percent sodium oxide, 2 percent alumina,
and 13 percent boric oxide. Such glass is about three times as heat-shock
resistant as soda-lime glass and is excellent for chemical and electrical uses.
This glass makes possible such products as ovenware and beakers, test tubes, and
other laboratory equipment.

Fused silica glass
glass is a highly heat-shock resistant glass that consists entirely of silica.
It can be heated to extremely high temperatures and then plunged into ice-cold
water without cracking. Fused silica is expensive because exceptionally high
temperatures must be maintained during production. It is used in laboratory
glassware and optical fibers.

96 percent silica glass
resists heat almost as well as fused silica, but it is less expensive to
produce. It consists of a special borosilicate composition that has been made
porous by chemical treatment. The pores shrink when the glass is heated, leaving
a smooth, transparent surface. The glass is sold under the trade name Vycor.

Colored glass
gets its coloring from certain oxides that are added to the glass. For example,
1 part of nickel oxide in 50,000 produces a tint that may range from yellow to
purple, depending on the base glass. One part of cobalt oxide in 10,000 gives an
intense blue. Red glasses are made with gold, copper, or selenium oxides. Other
colors can be produced in glass with other chemicals.